Rafter birdsmouth layout check

Birdsmouth Cut Calculator

Check rafter plumb angle and birdsmouth seat-cut depth from roof pitch, rafter depth, and the seat width you plan to mark. Use it as a conservative layout check before verifying code and structural details.

Units

Roof pitch (in / 12)Rafter depth (in)Seat cut width (in)Allowed notch check (%)Rafters to mark Waste / extra rafters (%)Cost per rafter ($)

Plumb cut angle

Seat notch depth

Max notch check

Rough cost

Birdsmouth cuts affect structural depth

This is geometry only. Allowable notch depth, bearing, rafter size, spans, loads, connectors, ridge details, and local code must be checked before cutting structural roof members.

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Rafter Cut Calculator

Review plumb, tail, and seat-cut sequence before marking boards.

Rafter Length Calculator

Calculate the common rafter length before laying out the seat.

Roof Pitch Calculator

Convert pitch to angle and multiplier for the same roof.

View all

Birdsmouth cut formula

Plumb angle = atan(pitch ÷ 12)

Seat notch depth = seat width × pitch ÷ 12

Max notch check = rafter depth × allowed notch %

Order cost = rafter count × rafter cost

Sample takeoff

A 6/12 roof has a 26.6° plumb angle. A 3.5 in seat cut drops about 1.75 in, while 25% of a 5.5 in rafter is 1.38 in, so the seat needs a structural check before cutting.A 6/12 roof has a 26.6° plumb angle. A 90 mm seat cut drops about 45 mm, while 25% of a 140 mm rafter is 35 mm, so the seat needs a structural check before cutting.

Birdsmouth Cut Calculator FAQ

How do I calculate a birdsmouth cut?+

Use pitch to get the plumb angle, then multiply the seat cut width by pitch divided by 12 to estimate the vertical notch depth.Use pitch to get the plumb angle, then multiply the seat cut width by pitch divided by 12 to estimate the vertical notch depth.

What is a safe birdsmouth depth?+

There is no universal answer. Many field checks use a fraction of rafter depth, but local code, engineering, span, and bearing details control the allowed notch.There is no universal answer. Many field checks use a fraction of rafter depth, but local code, engineering, span, and bearing details control the allowed notch.

Does this replace the rafter cut guide?+

No. Use this page for the seat-depth number, then use the rafter cut guide for the full cut sequence and safety reminders.No. Use this page for the seat-depth number, then use the rafter cut guide for the full cut sequence and safety reminders.